I think it's actually pretty hard to tell much from the membrane itself, my understanding is that most of the process is based in the membrane preferring the smaller water-only molecules and allowing the waste run off to carry away anything else. So they shouldn't really be catching much in the element.
They fail either from the chlorine degrading the membrane film which will start allowing larger dissolved solids through (your TDS will increase) or by having the pores scale and plug over time which will slowly decrease the output.
They fail either from the chlorine degrading the membrane film which will start allowing larger dissolved solids through (your TDS will increase) or by having the pores scale and plug over time which will slowly decrease the output.